1. Field of the Invention
The invention relates to methods of using transmitters and receivers to locate features of concealed objects and to circuitry for implementing such methods.
2. Description of the Prior Art
Telephone services are ordinarily provided by means of main telephone cable structures including a large number of conductors. Service to individual users is provided by means of individual telephone lines, referred to as "drops", connected to the main cable by means of "splices". Ordinarily, both the main cable and the "drops" are buried underground. Until recently, the main cables were periodically brought to the surface in metal boxes referred to as "enclosures". The "drops" were also brought to the surface in the enclosure closest to the user location. The installers would make the splices in the metal enclosures to connect "drops" for nearby users to appropriate conductors of the main telephone cable.
More recently, however, the metal enclosures are not utilized. Instead, a residential area to be served by the telephone system is premeasured. The cable manufacturing plant then accordingly measures the main cable and the "drop" conductors, makes the necessary splices, and seals or encapsulates the splices before the cable and "drops" are delivered to the residential location. The entire main cable, drop, and splice assembly is transported to the site and laid in previously excavated trenches. This method reduces costs by eliminating the need for metal enclosures and on-the-spot splice installation. However, splices have higher failure rates than other portions of the cable and drops, making it necessary to occasionally obtain access to splices in order to repair them. Presently it is difficult to identify the location of such buried splices.
One method of location underground point splices utilizes a large flat disc which contains a tuned conductive-capacitance circuit. The circuit has no active components. The disc is buried above the splice to be located. A high powered signal is transmitted into the ground near the location of the disc. This causes the buried tuned circuit in the disc to oscillate. The oscillation causes a signal to be radiated from the disc, wherefrom it is detected by a receiver which may be moved to maximize the intensity of the received signal, thereby indicating the location of the buried disc. Although this system has the advantage that no external power or control wires are required, the discs are large and inconvenient to use because they require careful alignment with the ground surface before being covered. The necessary precise alignment is often disturbed in the process of burying the cable and the discs. Further, the receiver must be carefully positioned on the opposite end of the long pole which supports the transmitter. The critical alignment of the disc unit and the precise spacing of the transmitter and receiver units on the pole causes the directly related transmitter signal to be cancelled in the receiver so that it detects only signals re-radiated from the underground tuned circuit. This requirement makes the detection assembly, including the transmitter and the receiver, very large and bulky and necessitates the above-mentioned critical alignment of the transmitter and receiver. A much lower cost, more convenient apparatus and method of detecting underground objects is needed.